The x-ray structure of the inward-facing conformation of a
LacY mutant (C154G) with a bound lactose homologue, -Dgalactopyranosyl-
1-thio--D-galactopyranoside (TDG), has
been solved recently (6). The overall structure reveals pseudotwofold
symmetry between the N- and C-terminal six-helix
domains, as proposed (19) for other members of the major
facilitator superfamily transporters. Similar symmetry has also
been found recently in the x-ray structure of GlpT (phosphate
glycerol phosphate antiporter), another member of the major
facilitator superfamily transporters (20). Importantly, the sugarbinding
site is found at the twofold axis of symmetry, which is
located approximately in the middle of the membrane, facing a
large water-filling internal cavity open to the cytoplasm (Fig.
1A). This finding is consistent with the idea that LacY has only
one binding site that has alternating accessibility to each side of
the membrane.
The x-ray structure of the inward-facing conformation of a
LacY mutant (C154G) with a bound lactose homologue, -Dgalactopyranosyl-
1-thio--D-galactopyranoside (TDG), has
been solved recently (6). The overall structure reveals pseudotwofold
symmetry between the N- and C-terminal six-helix
domains, as proposed (19) for other members of the major
facilitator superfamily transporters. Similar symmetry has also
been found recently in the x-ray structure of GlpT (phosphate
glycerol phosphate antiporter), another member of the major
facilitator superfamily transporters (20). Importantly, the sugarbinding
site is found at the twofold axis of symmetry, which is
located approximately in the middle of the membrane, facing a
large water-filling internal cavity open to the cytoplasm (Fig.
1A). This finding is consistent with the idea that LacY has only
one binding site that has alternating accessibility to each side of
the membrane.
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